Ensure there's a channel before trying to send on it

[arlolra]

Here's a trace on v0.12.9,

AssertionError: null == true
    at process.target._send (child_process.js:420:5)
    at process.<anonymous> (child_process.js:396:12)
    at process.emit (events.js:129:20)
    at handleMessage (child_process.js:324:10)
    at Pipe.channel.onread (child_process.js:350:11)

_disconnect nulls out the channel, but admits there may still be some buffering. If that's a NODE_HANDLE message, it'll attempt this ack. handleMessage has a similar check.

cc/ @inez

[mscdex]

Maybe it would be easier to backport #4418? That way there's less of a delta between v0.12 and modern branches. I'm kind of surprised it wasn't already backported really.

I wonder if v0.10 needs this as well....

[arlolra]

@mscdex definitely backport and abandon this. And it does look like v0.10 needs it as well.

[mscdex]

v0.12 should be fixed by #5153. I wasn't able to reproduce the problem on v0.10.

[arlolra]

Closing in favour of #5153